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Environmental dosimetry
Justin Brown
Assessing Risk to Humans and the Environment
NMBU, Ås, June 8th-9th 2017
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Lecture Contents
• Concepts and units
• Calculation of Absorbed fraction
• Internal and External DCCs
• ERICA Tool vs. Other tools
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Role of dosimetry in environmental assessment
The chart is reproduced from Jordi V. Batlle (2010).
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Key concepts: Kerma and absorbed dose
KERMA: Kinetic Energy Released per unit MAss
Kerma has the same units as absorbed dose but the absolute quantities
differ :
dm
dEK tr
Sum of the initial kinetic energies of all the charged particles liberated by
uncharged particles from ionizing radiation (e.g. protons, neutrons) in a unit
of mass:
especially for high energy photons and small volumes
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Absorbed dose
• Absorbed dose is the amount of energy deposited
by the ionising radiation to the material being
irradiated.
Louis Harold Gray
(1905 - 1965)
Only small amounts of deposited energy from ionising radiation are
required to produce biological harm – because of the means by which
energy is deposited (ionisation and free radical formation)
• The unit of absorbed dose is the Gray (Gy),
named after a British physicist.
• One Gray is one joule of absorbed energy
per kg of material. It is a relatively large unit
• Sub-multiples such as milligray (mGy), and
microgray (µGy) are often used
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Absorbed dose rate
• Non-human species have great differences in their average
lifetimes, so the most appropriate quantity in any dose
assessment is the estimation of dose rates (dose per unit time).
• If required, one can switch to doses by integrating the dose rate
over the lifespan or some other relevant period of the life of the
organism (e.g. the period of embryonic development).
• ERICA uses µGy/h but other units usually used are µGy/day or
mGy/day.
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Density of ionisation
• The radiation absorbed dose is a useful physical concept, but not
a good indicator of the likely biological effect
• Biological damage is not only dependent on the energy deposited
but also the density of ionisation;
a greater density of ionisation leads to a higher probability of DSB in DNA.
• Thus, we have to differentiate between alpha, beta and gamma
radiation as they differ from each other by penetrating capacity,
particle size, energy and by their ability to produce ions in
biological tissues.
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Linear Energy Transfer (LET)
• The average energy deposited along the track of a particle is
called the linear energy transfer (LET);
dx
dELET
• Alpha-particles ( together with some other less common forms of
ionizing radiation) are referred to as high-LET radiation, whereas
• Beta-particles and gamma-radiation are referred to as low-LET
radiation.
Defined by Bethe-Bloch expression
Function of particle charge, velocity of particle, mean
excitation potential of the target, atomic number etc.
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Relative Biological Effectiveness (RBE)
The greater efficiency of high-LET radiation in causing damage to
cells is commonly expressed in terms of the relative biological
effectiveness (RBE).
The RBE is defined as the ratio of dose required to achieve a specific biological
effect from a standard radiation (typically gamma rays) to the dose required for
the same end point from different types of radiation:
LEThigh
radiationref
D
DRBE
_
_
Humans: emphasis are on individuals and stochastic effects.
Plants and animals: more emphasis placed upon ’endpoints’ that are relevant for the integrity of the population – mortality, morbidity, reproduction effects
Important to note:
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RBE and radiation weighting factor (wr)
• The radiation weighting factors are defined as multipliers of absorbed dose used to account for the relative effectiveness of different types of radiation in inducing health effects.
• For humans, a-rad ”weighting” factor = 20 but this value is specific to stochasticeffects and therefore cannot be directlyused for plants and animals whereemphasis placed on populations
• RBE is dependent upon dose-rate, species, endpoint studied etc.
• The values of RBE can be experimentally estimated for different types of radiation, but
– it is practically impossible to obtain experimental values of RBE for a great number of possible endpoints and every type of organisms.
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Wr : Some suggested values
• UNSCEAR (1996) has proposed a radiation wr of 5 for alpha particles,
based on the approach that deterministic effects are of greater importance for
wildlife than stochastic effects.
• Systematic treatment of data Chambers et al. (2006) – up to 10 for deterministic, population relevant endpoints.
• A value of 20 for alpha particles is suggested in a number of
publications (e.g. Woodhead, 1984; Blaylock et al., 1993; Environment Agency, 2001).
• Based on experimental data, Kocher and Trabalka (2000) suggested that the
wrs for deterministic effects of alpha radiation are within the range from
5 to 10.
• Regarding (low energy) beta a value of 3 has been proposed for tritium
(Environment Canada, 2000; Environment Agency, 2001); UNSCEAR (1996)
has made a general recommendation to use a value of 1 for all beta
emitters.
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Wrs in ERICA
• 3 for low energy (≤ 10 keV) β
radiation
• 1 for γ and high energy (> 10 keV) β radiation
• 10 for α (non stochastic effects in the species)
vs. 20 for humans (to cover stochastic effects of radiation i.e. cancer in an individual)
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Equivalent dose (human radiological
protection)
• The product of absorbed dose with radiation
weighting factor (wr) is called Equivalent dose
• Unit of equivalent dose is Sievert (Sv)
• Called after the Swedish medical
physicist Rolf Sievert (1896 – 1966)
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Absorbed Fraction (AF)
Energies in the range from 10 keV to 5 MeV
Masses in the range from 1 mg to 1 tonne
ERICA assumes that the radiation sources are
uniformly distributed in spheres/ellipsoids
immersed in infinite aquatic medium and makes use of Monte
Carlo simulations to calculate absorbed fractions for photon and
electron.
The calculations for ERICA default geometries cover:
sourceby emittedEnergy
by target absorbedEnergy
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Interpolation: Photon sources in spheres
10-5
10-4
10-3
10-2
10-1
100
10-1
100
10-3
10-210-1
100101
102103
104105
106
AF
E (MeV)
Mas
s (g
)
Photon sources in spheres is a function of
Energy
Mass
Large mass and low
energy
1
Small mass, and High
energy,
0
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Absorbed fraction: Monoenergetic electrons
FASSET (2003) Dosimetric models and data for assessing radiation exposures to biota. FASSET (Framework for Assessment of
Environmental Impact) Deliverable 3 Report under Contract No FIGE-CT-2000-00102, G. Pröhl (Ed.).
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Absorbed fraction: Monoenergetic γ-radiationW
ide ra
nge
FASSET, (2003) Dosimetric models and data for assessing radiation exposures to biota. FASSET (Framework for Assessment of
Environmental Impact) Deliverable 3 Report under Contract No FIGE-CT-2000-00102, G. Pröhl (Ed.).
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Internal dose ratesHomogeneous distribution in organism
Dose rates for internally incorporated radionuclides. The dose rates delivered
to organisms are evaluated from the concentration of each internally
incorporated radionuclide.
1
14
ii
i
iTorgint
kg.Bq.MeV
µGyh1076.5k
yE)E(AkD
where:
Aorg is the activity concentration in organism (Bq kg-1 w.w.);
Ei is the energy of component <i> of emitted radiation (MeV);
yi is the yield of emitted radiation of energy Ei (dis-1);
T(Ei) is the absorbed fraction in the target for energy Ei;
K is the factor to account for conversions of MeV to Joules and seconds to hours.
For each radionuclide data for the energy and yield of particle, photon and a particle
emissions have been extracted from the literature (ICRP, 1983).
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External dose ratesHomogeneous distribution in the environmental media
• Dose rates for external exposure from radionuclides present in soil or
in water column are calculated using a variant of the simple formula
for uniformly contaminated isotropic infinite absorbing medium:
1
14
ii
i
iTenvExt
kg.Bq.MeV
µGyh1076.5k
yE))E(1(AkD
Aenv is the activity concentration in the environment in Bq kg-1 or Bq m-3.
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Dose Conversion Coefficient (DCC)
• Defined as the ratio of dose rate per unit activity concentration in
organism or the medium:
org
intint
A
DDCC
env
extext
A
DDCC
Units of µGyh-1 per Bq kg-1
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A short summary
The dose is the result of a complex interaction of energy, mass and the
source – target configuration:
Only a few organisms with simple geometry can be simulated explicitly-
Reference organisms
Define organism mass and shape
Consider exposure conditions (internal, external)
Derive absorbed fractions: Simulate radiation transport for mono
energetic photons and electrons.
Calculate Dose Conversion Coefficients: Link calculations with
nuclide-specific decay characteristics
In all other cases interpolation has to be applied
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Reference organisms
The enormous variability of biota requires the definition of reference
organisms that represent:
Plants and animals
Different mass ranges
Different habitat
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Reference organism: Assumptions
• Homogeneous distribution of radionuclides within the organism
• Organism immersed in uniformly contaminated medium
• Organs are not considered
• Dose rate averaged over organism volume
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Interpolation method for ellipsoids
• In order to consider absorbed fractions for a wide range of different
ellipsoid shapes Rescaling Factors were used (Ulanovsky and Pröhl,
2006):
Ulanovsky, A., Pröhl, G., 2006. A practical method for assessment of dose conversion coefficients for aquatic biota. J.
Environ. Biophys. 45, 203–214.
),(
),,(),,(
0 ME
MEMERF
The differences between absorbed fractions for spherical and non-spherical
bodies have been found to depend on mass and shape of the body, as well as
on type and energy of the source particles.
• RF expressed as a function of a ”non-sphericity parameter” η
Defined as the ratio of the surface area of a sphere to that of a non-
spherical body of equal mass
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Representative situations
• The number of possible situations is enormous; therefore a
limited number of representative situations have been selected
for detailed calculations:
In soil/ on soil/ in air
In water/ interface water-air
In sediment/ interface water-sediment
Exposure target Radiation source
Air Soil Water Sediment
Air × ×
Soil surface × ×
Soil × ×
Water × ×
Sediment surface × ×
Sediment × ×
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Assumed compositions
Element
Material
Organism tissue Shielding layer Soil Air
H 10.2 7 2.1 0.064
C 14.3 50 1.6 0.014
N 3.4 16 75.09
O 71.0 24 57.7 23.56
Na 0.1
Al 5.0
Si 27.1
P 0.2
S 0.3 3
Cl 0.1
Ar 1.28
K 0.4 1.3
Ca 4.1
Fe 1.1
Density (g/cm3) 1.05 1.0 1.6 0.0012
The density of the materials involved and their elemental composition
have an important impact on the radiation transport.
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Photon mean free path vs. composition
The mean free path of photons is plotted for air, water, tissue and soil
(density = 1.6 g/cm³) as function of the energy
FASSET (2003) Dosimetric models and data for assessing radiation exposures to biota. FASSET (Framework for Assessment of
Environmental Impact) Deliverable 3 Report under Contract No FIGE-CT-2000-00102, G. Pröhl (Ed.).
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Monte Carlo approachSimulations of photon and electron transport through matter
• Materials differing in composition and density can be considered;
• Complex geometries of sources and targets can
be simulated;
• All relevant physical processes that
control radiation transport are
precisely treated;
• Self-shielding is implicitly considered.
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Problems and limitations
Monte Carlo calculations are very time-consuming:
Long range of high-energy photons in air, a large area around the
organism has to be considered
A large contaminated area has to be considered as source
Small targets get only relatively few hits
Probability of a target being hit is inversely proportional to the square of the
distance between source and target
Simulations require high number of photon tracks
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Biota living on the ground
• The external DCCs have been calculated here as a product of
free-in-air kerma, Ka, in a place occupied by the animals’ body
and pre-computed dose-to-kerma ratios, R(Ei,M):
i
i
iiaext y)M,E(R)E(KDCF
Taranenko, V., Pröhl, G., Go´mez-Ros, J.M., 2004. Absorbed dose rate conversion coefficients for reference biota for
external photon and internal exposures. J. Radiol. Prot. 24, A35–A62.
• It is assumed that only photon sources contribute to external
exposure of the animals.
• A volume source uniformly distributed to a depth of 10 cm has
been assumed
Ei is the energy , yi is the
yield of specific photon (per
decay), and M is the mass of
the animal (kg).
The Monte Carlo calculations were performed for 19 energies in the range from
10 keV to10 MeV.
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Biota living in the ground
• It is assumed that the target organism is at a depth of 25 cm in a
uniformly contaminated soil layer of 50 cm
• Hence the Monte Carlo model geometry
consists of soil compartment which is the
isotropic source and the target in the
middle as receptor
• The Monte Carlo simulations have been
performed for seven initial energies in
the range from 10 keV to 3 MeV
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Locations within habitat
Terrestrial FreshwaterMarine
Fraction of time spent by organism at different locations
within its habitat
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Calculation of dose rates
• Internal exposure:
• External exposure:
intintint )( DCCCDCCCRCD orgnismorganismmediummedium
occupancyextmediumext fDCCCD
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Note on DCCs
We also normally interested in weighted total dose rates (in µGy/h)
Where:
wf = weighting factors for various components of radiation (low beta, + and alpha)
DCC = Dose Conversion Coefficient in µGy/day per Bq/L or kg
aa
,,
int,int,int,int
extlowextlowext
lowlow
DCCwfDCCwfDCC
DCCwfDCCwfDCCwfDCC
Need to apply : Radiation weighting factors (dimensionless):
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IAEA intercomparisons : dosimetric models (1)
• Models have been developed for specific situations in various countries
• International comparison of 7 models performed under the IAEA EMRAS I program: EDEN, EA R&D 128, DosDimEco, EPIC-Doses3D, RESRAD-BIOTA, SUJB and ERICA
• 5 ERICA runs by different users
• DCCs compared for 67 radionuclides and 5 ICRP RAP geometries
– Exploratory stats – central tendency, skewness, outliers etc.
– ‘‘Z-score’’, which is a measure of how many standard deviation units away from the mean a particular data value lies
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IAEA intercomparisons: dosimetric models (2)
• Greater variation for external exposure DCCs, explained by
Vives i Batlle J., Balonov M., Beaugelin-Seiller K., Beresford N. A., Brown J., Cheng J-J., Copplestone D., Doi M., Filistovic V., Golikov V.,
Horyna J., Hosseini A., Howard B. J., Jones S. R., Kamboj S., Kryshev A., Nedveckaite T., Olyslaegers G., Pröhl G., Sazykina T.,
Ulanovsky A., Vives Lynch S., Yankovich T. and Yu C. (2007). Inter-comparison of absorbed dose rates for non-human biota. Radiation and
Environmental Biophysics, 46, 349-373.
• DCCs for internal exposure similar for different approaches
– Number of daughter products being included in the DCC of the parent
– Differences in assumptions for media densities (particularly to low-energy -emitters)
ERICA made predictions similar to other models
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Inhomogeneous distributions
• In the absence of more detailed information, the whole body dose
rate due to internal exposure in reference organisms have been
calculated using:
The average whole body activity concentration and
• Realistic scenarios of internal exposure must account for some
radionuclides which tend to concentrate in specific organs or
tissues.
The DCCs values obtained assuming a homogeneous distribution.
• To study the effect of such inhomogeneous distributions, internal
DCCs have been calculated assuming both a central and an
eccentric point source (J.M. Gomez-Ros et al., 2008).
J.M. Gomez-Ros et al. (2008). Journal of Environmental Radioactivity, 99, 1449 -1455
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A note on combining DCCs in ERICA
• Any given decay chain is truncated
when the physical half-life of a given
daughter product exceeds 10 days.
The DCCs of all progeny up to that
point are then combined with the
parent radionuclide by assuming that
the entire group of radionuclides is in
secular equilibrium.
• The use of a 10 day cut-off is
somewhat arbitrary as is the
assumption that parent and daughter
will be at equilibrium
– More correct to consider decay and
ingrowth over environmentally relevant
integration periods
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Conclusions
• ERICA makes many assumptions and simplifications but it is well
elaborated and has been verified through international comparison
• There are some things ERICA cannot do:
• Limitations with regards to selection of habitat for reference
organisms e.g. cannot calculate DCC for marine bird in air
• Gaseous radionuclides are beyond the scope of the tool and
require specialised models
• Dosimetry of plants have to be improved; do not represent
whole-organism
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Future developments
Progression in human dosimetry towards voxel
phantoms provides a clear indication of the way
forward for improving the dosimetry for animals.
http://nsed.jaea.go.jp/ers/radiation/en/rpro/mouse-e.htm